This invention relates to a structure of an optical fiber submarine cable for use in the field of light communication employing a low-loss optical fiber as a transmission medium.
Since an optical fiber is very brittle it is usually coated with nylon, polyethylene or the like when put to practical use. In a case of the coated optical fiber being employed in a submarine cable, if seawater pressure is applied to the coated optical fiber, it causes an increase in transmission loss, making impossible the effective use of the low-loss property which is characteristic of the optical fiber; furthermore, exposure of the coated optical fiber to the seawater pressure is also undesirable from the view-point of reliability of the optical fiber. To avoid this, there has been proposed an optical fiber submarine cable of such a construction that a coated optical fiber is housed in a small-diametered, cylindrical pressure resisting structure so as to protect the coated optical fiber from seawater pressure. In such a cable, however, when applied with seawater pressure, the inner diameter of the pressure resisting structure is reduced, though slightly, resulting in some pressure being applied to the coated optical fiber through a material present between the coated optical fiber and the pressure resisting structure. Accordingly, a cable structure of the type that oil or plastic material is filled in the space defined between the inner wall of the pressure resisting structure and the coated optical fiber allows application of a pressure of about 1/10 of the seawater pressure to the coated optical fiber and hence is not preferred. In a case where the coated optical fiber is not closely packed but housed in the pressure resisting structure with a suitable air gap defined therebetween, no pressure is applied to the coated optical fiber to such an extent as to affect the transmission loss by the reduction of the inner diameter of the pressure resisting structure due to the seawater pressure applied thereto because the compressibility factor of air is larger than that of a liquid or solid material. In such a cable structure in which the air gap exists between the coated optical fiber and the pressure resisting structure, however, if the cable is damaged by fishing implements, an anchor of a vessel or the like, then seawater enters into the pressure resisting structure from the damaged part, exposing the coated optical fiber to seawater over substantially the entire length of one repeater section. The principal component of the coated optical fiber is usually high purity quartz; in a humid atmosphere such as in the sea, there is a fear that the strength of the material of the optical fiber is lowered so as to result in breakage of the optical fiber. Further, quartz itself is easily affected by sodium ions. Therefore, if seawater enters into the pressure resisting structure as a result of damage inflicted on the cable, as mentioned above, then the mechanical properties of the optical fiber are adversely affected. Moreover, if the cable is repaired with the seawater left remaining in the pressure resisting structure, then the remaining seawater and undulations of the bottom of the sea cause a non-uniform pressure to be applied to the optical fiber to degrade its transmission characteristic, so that a submarine cable transmission system of high reliability and good quality cannot be maintained, and the optical fiber submarine cable must be replaced by a new one over one repeater section; since the cost and labor for repair of injury are enormous, the above said cable structure is very uneconomical. Accordingly, it is important to design a cable in a manner to minimize the possibility of the coated optical fiber being exposed to seawater when the cable is damaged.